This invention relates to the art of processing video signals and, more particularly, to separating synchronizing signals from a video signal wherein the slicing level of the sync separating circuitry is varied as a function of noise in a nonpicture portion of a video signal.
Sync separators circuits are known in the art for recovering synchronizing information from a composite video signal. The composite video signal typically includes periodic nonpicture portions, as well as picture information. The nonpicture portions may include, for example, a front porch portion, a sync tip portion and a back porch portion. The front porch portion and the back porch portion may have the same DC level and straddle the sync tip portion which is at a different DC level, such as a more negative level with respect to the other portions. The recovery circuit may include a sync slicer which serves to compare the incoming video signal with a threshold known as the slicing level. So long as the input signal is more negative than the slicing level, a sync signal is provided. The slicing level is typically chosen so as to be midway between that of the front porch or back porch portion and that of the sync tip portion. However, in the presence of noise on the video signal, the sync slicer may erroneously provide an output sync signal when the noise excursions cross the slicing level. Consequently, it is desirable to provide such a sync separator circuit with a slicing level which varies to compensate for noise that may be present on the video signal to prevent erroneous generation of an output sync signal.
It is known in the art to provide sync separator circuits having means for providing variable slicing levels. Such are found, for example, in the U.S. Pat. Nos. to J. R. Harford, 4,185,299, and M. McGinn, 4,357,629. The sync separator circuits in these patents are provided with means for varying the slicing level with variations in the magnitude of the input video signal. However, neither McGinn nor Harford provides circuitry for varying the slice level as a function of measured noise. Harford suggests that the recovered sync signal undergo wave shaping or filtering in the presence of noise.